The present invention relates to a cache control technique and an information processing technique, and more particularly to technique which is effective in providing efficient operation of a cache memory provided in a control unit interposed between a control processing unit and an external storage of an information handling system. It also relates to a technique which is suitable for loading of data into the cache memory.
The inventors of this invention filed a Japanese Patent Application now published as JP-A-60-196860.
A magnetic disk device which is used as the external storage has an advantage of realizing a large storage capacity with a relatively low cost. However, due to the waiting time for rotation of a magnetic disk which serves as the storage medium and a seek operation of the head, there is a limit to the improvement of the accessing speed which may be achieved in recording and reproducing data.
In a computer system which uses a magnetic disk device as an external storage, a cache memory which comprises a semiconductor memory capable of being accessed faster than the magnetic disk device is provided in a magnetic disk control unit which is interposed between a central processing unit and the magnetic disk device so that the magnetic disk control unit may store a portion of the data stored in the magnetic disk device and respond to an access from the central processing unit. In this manner, the large difference between the operation speeds of the magnetic disk device and the central processing unit is compensated. It is a usual practice to make the exchange of data between the magnetic disk device and the central processing unit efficient.
In such a system, it is important from a standpoint of more efficient function of the cache memory to increase the probability that desired data accessed from the central processing unit is present in the cache memory, that is, that the hit rate is as high as possible. To this end, the following various techniques have been proposed heretofore.
In the technique disclosed in JP-A-55-154648, a storage area of the magnetic disk device is divided into a plurality of areas. A different operation mode is preset for each of the divided storage areas, and the operation mode corresponding to the storage area designated by an input/output command from the central processing unit is selected. In this manner, the hit rate is improved.
In the technique disclosed in JP-A-60-14360, statistical data for hit and miss are collected by the disk control unit. The collected statistical data is transferred to the central processing unit. Based on the statistical data, an operator or a system manager determines whether the current utilization status of the cache memory is optimum or not, and based on the result of determination, the extent of data to be loaded from the magnetic disk device to the cache memory, that is, the extent of cache loading, is appropriately controlled. In this manner, the transfer efficiency of the data between the central processing unit and the magnetic disk device is improved.
In the paper of the 29th Joint Conference of the Information Processing Society of Japan, pp. 169-170, it is proposed that the disk control unit detects a sequential access, and after the sequential access to a track has been completed, that track is treated as data which may be purged from the cache memory at a highest priority.